Exercise effects, lactic acid and oxygen debt
Bioenergetics • Respiration
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Immediate responses to exercise
Heart rate increases so that blood delivers oxygen and glucose to muscles more quickly; blood vessels supplying active muscles dilate, increasing local blood flow and oxygen delivery . Breathing rate and breath volume increase, raising the rate of oxygen uptake at the lungs and the removal of carbon dioxide, which supports an elevated aerobic respiration rate . Cardiac output rises because both heart rate and stroke volume increase, allowing more blood per minute to reach tissues.
Cause of lactic acid buildup
When oxygen delivery to muscle cells is too low for the required rate of aerobic respiration, cells respire anaerobically. Anaerobic respiration converts glucose to lactic acid and releases only a small fraction of the energy compared with aerobic respiration (around 5%), causing a rapid accumulation of lactic acid in muscle cells . High lactic acid concentration alters local pH and contributes to muscle pain, cramp and reduced contractile efficiency.
Oxygen debt and recovery (definition and reaction)
Oxygen debt describes the extra oxygen required after vigorous activity to oxidise accumulated lactic acid and to restore normal resting conditions (HT only) . During recovery, continued deep and rapid breathing supplies this extra oxygen. Lactic acid reacts with oxygen to form carbon dioxide and water, releasing the remaining energy stored in the lactic acid; this completes metabolism of the glucose fragments produced during anaerobic respiration .
Transport of lactic acid to the liver (HT only)
Lactic acid diffuses from muscle cells into the blood down a concentration gradient. Blood transports lactic acid to the liver, where it diffuses into liver cells and is converted back into glucose by oxidative reactions. The newly formed glucose then returns to the blood for storage as glycogen or for use by tissues, completing the recovery process .
Muscle fatigue and limiting factors
Prolonged vigorous activity causes muscle fatigue through several mechanisms: depletion of muscle glycogen stores reduces available substrate for respiration; accumulation of lactic acid lowers pH and impairs enzyme function and contractile proteins; and reduced oxygen supply forces sustained anaerobic respiration, which provides little energy per glucose molecule . Recovery depends on oxygen availability, circulatory transport, and metabolic capacity of the liver and muscles.
Key notes
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